Hemihydrate gypsum (stucco) is commonly produced from gypsum ore by drying, grinding and calcining it. The calcination step is employed to convert the dihydrate gypsum to the hemihydrate gypsum as shown in the forward reaction:CaSO4.2H2O+heat→CaSO4.½H2O+ 3/2H2O.
The above reaction is reversible. When water is added to the hemihydrate gypsum the reverse reaction occurs resulting in dihydrate gypsum.
One common application of gypsum is the manufacture of wallboard. The amount of water necessary to produce the required fluidity to manufacture wallboards exceeds the amount of water necessary to convert the hemihydrate to the dihydrate gypsum. The excess water increases the time required for the slurry to set and the time required for the product to dry in a kiln.
One method of reducing the amount of water and improving the fluidity in a gypsum composition is to add a polymeric dispersant. For example, U.S. Pat. No. 6,527,850 discloses the use of a comb-branched copolymer in gypsum compositions to improve the fluidity. Similarly, U.S. Pat. Nos. 6,869,988 and 7,070,648 respectively disclose the addition of a supported acrylic/polyether comb-branched copolymer and a sulfonate-containing comb-branched copolymer to gypsum compositions to improve the fluidity.
While gypsum rock or ore is predominantly gypsum, it may also contain clay. We have observed a negative interaction between the clay and the comb-branched copolymer dispersant. This results in the copolymeric dispersants becoming less effective. It would be commercially useful if polymeric dispersants retain their effectiveness in the presence of clay.